From: Paul Mackerras Date: Sat, 14 May 2022 01:59:14 +0000 (+1000) Subject: FPU: Minor fix and simplifications X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=03ca9b5dd5ef466f0a263ac2e9caaeb4310a0d76;p=microwatt.git FPU: Minor fix and simplifications In preparation for an explicit exponent data path. The fix is that fre[s] needs to negate the exponent after renomalization rather than before, otherwise the exponent adjustment done by the renormalization is in the wrong direction. Signed-off-by: Paul Mackerras --- diff --git a/fpu.vhdl b/fpu.vhdl index c06c4bd..d838872 100644 --- a/fpu.vhdl +++ b/fpu.vhdl @@ -56,7 +56,7 @@ architecture behaviour of fpu is ADD_1, ADD_SHIFT, ADD_2, ADD_3, CMP_1, CMP_2, MULT_1, - FMADD_1, FMADD_2, FMADD_3, + FMADD_0, FMADD_1, FMADD_2, FMADD_3, FMADD_4, FMADD_5, FMADD_6, LOOKUP, DIV_2, DIV_3, DIV_4, DIV_5, DIV_6, @@ -1538,9 +1538,9 @@ begin v.fpscr(FPSCR_FR) := '0'; v.fpscr(FPSCR_FI) := '0'; v.use_b := '1'; + v.result_exp := r.b.exponent; case r.b.class is when FINITE => - v.result_exp := - r.b.exponent; if r.b.mantissa(UNIT_BIT) = '0' then v.state := RENORM_B; else @@ -1600,7 +1600,7 @@ begin -- else AIN_B v.result_sign := r.a.negative; v.result_class := r.a.class; - v.result_exp := r.a.exponent; + v.result_exp := r.a.exponent + r.c.exponent; v.fpscr(FPSCR_FR) := '0'; v.fpscr(FPSCR_FI) := '0'; v.use_a := '1'; @@ -1610,8 +1610,6 @@ begin if r.a.class = FINITE and r.c.class = FINITE and (r.b.class = FINITE or r.b.class = ZERO) then v.is_subtract := not is_add; - mulexp := r.a.exponent + r.c.exponent; - v.result_exp := mulexp; -- Make sure A and C are normalized if r.a.mantissa(UNIT_BIT) = '0' then v.state := RENORM_A; @@ -1627,15 +1625,10 @@ begin -- addend is bigger, do multiply first v.result_sign := not (r.b.negative xor r.insn(1) xor r.insn(2)); f_to_multiply.valid <= '1'; - v.state := FMADD_1; + v.state := FMADD_0; else - -- product is bigger, shift B right and use it as the - -- addend to the multiplier - v.shift := r.b.exponent - mulexp + to_signed(64, EXP_BITS); - -- for subtract, multiplier does B - A * C - v.result_sign := not (r.a.negative xor r.c.negative xor r.insn(2) xor is_add); - v.result_exp := r.b.exponent; - v.state := FMADD_2; + -- product is bigger, shift B first + v.state := FMADD_1; end if; else if r.a.class = NAN or r.b.class = NAN or r.c.class = NAN then @@ -1715,11 +1708,7 @@ begin when RENORM_B2 => set_b := '1'; - if r.is_sqrt = '0' then - v.result_exp := r.result_exp + r.shift; - else - v.result_exp := new_exp; - end if; + v.result_exp := new_exp; v.opsel_a := AIN_B; v.state := LOOKUP; @@ -1830,7 +1819,7 @@ begin v.state := FINISH; end if; - when FMADD_1 => + when FMADD_0 => -- Addend is bigger here v.result_sign := not (r.b.negative xor r.insn(1) xor r.insn(2)); -- note v.shift is at most -2 here @@ -1844,6 +1833,15 @@ begin v.state := ADD_SHIFT; end if; + when FMADD_1 => + -- product is bigger here + -- shift B right and use it as the addend to the multiplier + v.shift := r.b.exponent - r.result_exp + to_signed(64, EXP_BITS); + -- for subtract, multiplier does B - A * C + v.result_sign := r.a.negative xor r.c.negative xor r.insn(2) xor r.is_subtract; + v.result_exp := r.b.exponent; + v.state := FMADD_2; + when FMADD_2 => -- Product is potentially bigger here -- r.shift = addend exp - product exp + 64, r.r = r.b.mantissa @@ -1993,6 +1991,7 @@ begin v.state := FINISH; when FRE_1 => + v.result_exp := - r.result_exp; opsel_r <= RES_MISC; misc_sel <= "0111"; v.shift := to_signed(1, EXP_BITS);